Ceramic enamel compositions can be used for a variety of applications, such as decorative coatings for glassware, chinaware, and the like. They are especially useful in forming colored borders around glass sheets used for automotive windshields, sidelights and backlights. The colored borders enhance appearance as well as prevent degradation of underlying adhesives by UV radiation.
In general, these enamel compositions consist mainly of a glass frit, a colorant and an organic vehicle. They are applied to a desired region of the substrate and subsequently fired to burn off the organic vehicle and fuse the ceramic solids to the surface of the substrate.
Glass sheets for automotive use are generally coated in the desired region with the ceramic enamel composition and then subjected to a pressure forming process at elevated temperatures. During this treatment the enamel melts and fuses to the glass substrate and the glass is formed into a desired final shape. However, many previous coatings exhibit a tendency to adhere to the materials covering the forming die, e.g., a fiberglass or metal fiber covered die, because these conventional enamels have a low viscosity after melting and tend to stick to other materials at high temperature. Accordingly, such previous enamels are not suitable for use in glass forming processes in which the heated glass coated with enamel is pressure formed with a die.
Various approaches have been suggested in order to facilitate the forming of glass sheets with a ceramic enamel coated thereon without the enamel adhering to the forming die. For example, U.S. Pat. Nos. 4,596,590 and 4,770,685 (issued to Boaz) propose the addition of a low valent metal oxide powder, e.g., cuprous oxide, to the paint composition to provide a non-stick barrier between the coating and the fiberglass-covered forming die. U.S. Pat. Nos. 4,684,389; 4,857,096; 5,037,783 and EP 490,611 (issued to Boaz), propose adding finely divided zinc metal powder for a similar effect. The use of an iron metal powder is proposed in U.S. Pat. No. 4,983,196 (issued to Stotka).
A purportedly improved anti-stick ceramic enamel composition is proposed by U.S. Pat. Nos. 5,153,150; 5,208,191 and 5,286,270 (issued to Ruderer et al.) wherein a seed powder containing Zn.sub.2 SiO.sub.4 is combined with the glass frit portion of the composition. The glass frit portion comprises at least 35 percent by weight precursors for crystalline Zn.sub.2 SiO.sub.4, more particularly, at least 30 weight percent ZnO and at least 5 weight percent SiO.sub.2.
A further shortcoming of a number of previous ceramic enamel systems is that they employ a lead-containing glass frit. For environmental considerations it is desirable to avoid the use of any lead-containing system.
Along these lines, U.S. Pat. No. 4,882,301 (issued to Gettys et al.) proposes use of a crystallizing amount of Cd.sub.2 SiO.sub.4 with a lead borosilicate glass. This reference states that Zn can be substituted directly for Cd in the glass formulation; however, U.S. Pat. No. 5,208,191 indicates that when zinc is substituted for cadmium, the results have been less than desirable.
Also, while several of the above-mentioned enamel systems may perform satisfactorily in conventional glass forming processes, some may not be suitable for use in the newly-developed "deep bend" processes for forming automotive glass. Moreover, the enamel compositions must resist certain chemical agents which they may contact.
The previous enamel compositions suffer from one or more of the deficiencies noted above. In contrast, the present invention provides a ceramic enamel composition that avoids these shortcomings.